U.S. Sen. Mark Udall and Rep. Cory Gardner during a debate Tuesday, Oct. 7, 2014 in the auditorium of The Denver Post in Denver, Colo.

Colorado candidates for U.S. Senate, Senator Mark Udall and challenger, Representative Cory Gardner shake hands backstage before taking the stage for a debate Tuesday, October 7, 2014 in the auditorium of The Denver Post in Denver, Colorado.

A new Denver Post poll shows Colorado’s marquee U.S. Senate race is shifting toward Republican Cory Gardner, as President Barack Obama’s sagging popularity erodes the advantage once held by Democratic U.S. Sen. Mark Udall.

Gardner and Udall remain in a tight race, 45 percent to 43 percent, according to a SurveyUSA poll of likely voters released Monday.

Gardner’s lead is within the margin of error, making the race a statistical tie, but it represents a reversal from a month ago when Udall held a 4-point edge.

A 10-point lead among independents in a September poll that favored Udall disappeared. It’s now tied at 40 percent, the poll found. And a 13-point gap among woman that favored Udall is now seven points.

Both shifts came despite Democratic efforts to target these key swing voters by emphasizing social issues and painting Gardner as too conservative on abortion and birth control.

On the other side, Republicans attacks on Udall appear to be weakening his popularity. Udall’s job approval is 36 percent compared to 53 percent disapproval, a 17-point negative gap, the poll found. A month ago his approval rating stood at negative six.

By contrast, Gardner is only becoming more popular. A month ago voters were evenly split on him. Now, his favorability stands at 43 percent and unfavorability at 38 percent.

Gardner’s momentum comes at a key moment in the campaign as the state prepares to mail ballots to voters Tuesday.

In a High Point University poll released Monday, Gardner held a 4-point lead with 46 percent support compared to 42 percent for Udall, an advantage outside the margins. The poll, conducted Oct. 4-8 by SurveyUSA, used different methodology.

One factor evident in both surveys is how Obama’s job approval rating is hurting Udall’s chances.

In The Post poll, two-thirds of likely voters polled suggest their feelings about Obama are a factor in how they will vote in November. Of those who consider the president a major factor, 68 percent favor Republicans.

“What you have is an anti-Obama sentiment top of mind, front lobe, for many of the Republicans who are showing up to vote in 2014,” Leve said. “By contrast when you ask a Democrat how much the president is affecting or influence their vote in 2014, they respond, ‘Obama who?’ “

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